Primarily because of their low transition loss, optical fibers, which comprise a cylindrical glass core surrounded by a cladding layer, have become the dominant medium for transmitting information in the form of light. In time, it is foreseen that optical fibers will be used for transmitting light over relatively short distances much as metal conductors presently transmit electrically in electronic circuits. As such time, there will be a great need for inexpensive optical couplers that can controllably remove light from, or apply light to, an optical fiber.
An important step in the fabrication of such optical couplers or optical fiber taps is the controlled removal of part of the cladding to expose part of the core of the optical fiber. The size, shape, position, and cleanliness of the cladding removal is critical in making the coupler; it is also important that the optical fiber core not be damaged mechanically or optically during the cladding removal and that the fabrication method be reasonably convenient to perform. Various mechanical stripping methods and chemical etching methods have been proposed. These methods are suitable for laboratory purposes, but have not proven amenable to mass production because of the difficulty of obtaining a suitably high yield, that is, the difficulty of obtaining a high proportion of usable devices while avoiding serious damage to the optical fiber.
The copending application of Coyle et al., Ser. No. 454,603, filed Dec. 21, 1989, hereby incorporated by reference herein, describes a method for using an excimer laser to remove a cladding portion of an optical fiber by ablation while minimizing damage to the core. The laser produces a beam of ultraviolet light that is absorbed by the cladding but transmitted efficiently by the core so that damage to the core is minimized. A drawback of the invention is that the laser beam is transmitted transversely through the optical fiber core and impinges on the cladding on the side of the optical fiber opposite the laser. It is desired that cladding be removed from only one well defined area, and destruction of the cladding on the opposite side can be quite harmful since it may lead to spurious leakage of optical energy. The Coyle et al. application solves this problem by including on the side of the optical fiber opposite the opening an assembly for containing repair media that can be used to repair such damage. This requirement, of course, further complicates the fabrication of the coupler.
It has been recognized that it would be preferable to control the laser machining of the cladding so as to avoid serious damage to the cladding intended to be left intact. The cladding on optical fibers is not always uniform, and so the Coyle et al. technique can sometimes cause more damage than predicted. In sum, there has been a long-felt need for a method to strip controllably a cladding portion of an optical fiber in a manner that is amenable to mass production, is highly reliable, which does not require greater operator skill, and which does not depend on optical fiber uniformity, particularly of the cladding thickness.